The use of drop ceilings in building construction is well known. One mode of construction for drop ceilings is to provide a metal framework with longitudinal runners and lateral cross-members which fit together in a lattice network. Standard openings have modular sizes of 2' .times. 2' squares or 2' .times. 4' rectangles. The modular framework is supported on hangers from the overhead structure and the framework is used to support acoustical tile panels, fluorescent light fixtures, ventilation fixtures, and the like.
In the past, both steel grid systems and extruded aluminum grid systems have been available. The steel grid systems have been constructed from lengths of T-shaped members which are roll-formed out of sheet steel stock. LSince the acoustical panels rest on top of the inverted T frame members, the bottom surface of the T is offset from the surface of the acoustical panels. This is not particularly desirable where movable partition type walls are used, since a gap is formed between the top of the partition and the surface of the acoustical panel to allow for the thickness of the inverted T-shaped member. Moreover the flat surface of the cross portion of the T that is exposed is relatively wide to achieve stiffness and because of its width does not make a particularly pleasing appearance in outlining the grid structure of the drop ceiling.
Drop ceiling frameworks have also been provided which are made from extruded aluminum. Interlocking of aluminum frame joints presents a somewhat different problem in that either separate clips must be provided for connecting the parts or interlocking integral tabs provided that must be bent to lock the parts together. Aluminum extrusions have the additional disadvantages in that they are not fire resistive and cannot be fire rated. Also, if contrasting tones are desired, the aluminum must be painted after fabrication, by masking techniques, greatly adding to the cost. Thermal expansion is a problem in large floor plans.